Apparatus and method for communicating data over an optical channel

ABSTRACT

An optical module processes first FEC (Forward Error Correction) encoded data produced by a first FEC encoder. The optical module has a second FEC encoder for further coding a subset of the first FEC encoded data to produce second FEC encoded data. The optical module also has an optical modulator for modulating, based on a combination of the second FEC encoded data and a remaining portion of the first FEC encoded data that is not further coded, an optical signal for transmission over an optical channel. The second FEC encoder is an encoder for an FEC code that has a bit-level trellis representation with a number of states in any section of the bit-level trellis representation being less than or equal to 64 states. In this manner, the second FEC encoder has relatively low complexity (e.g. relatively low transistor count) that can reduce power consumption for the optical module.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.16/853,514 filed Apr. 20, 2020, which is a continuation of U.S. patentapplication Ser. No. 16/451,986 filed Jun. 25, 2019, now U.S. Pat. No.10,659,192 issued May 19, 2020, which is a continuation of U.S. patentapplication Ser. No. 16/155,610 filed Oct. 9, 2018, now U.S. Pat. No.10,374,750 issued on Aug. 6, 2019, which is a continuation of U.S.patent application Ser. No. 15/494,366 filed Apr. 21, 2017, now U.S.Pat. No. 10,128,980 issued on Nov. 13, 2018, which is a continuation ofU.S. patent application Ser. No. 14/976,100 filed on Dec. 21, 2015, nowU.S. Pat. No. 9,654,253 issued on May 16, 2017, the entire disclosuresof which are incorporated by reference.

FIELD OF THE DISCLOSURE

This application relates to communication systems, and more particularlyto communicating data over an optical channel.

BACKGROUND

An optical module of a communication system can send and/or receive dataover an optical channel. The data can be coded by one or more FEC(Forward Error Correction) encoders prior to transmission over theoptical channel. Such coding is performed because the optical channeldistorts and adds noise to the transmitted data. The coding can enablethe data to be recovered at a receiver even in the presence ofimpairments.

Performance of the coding can be determined based on whether the datacan be recovered at the receiver. FEC codes with complex encoders and/ordecoders can often achieve greater performance than codes with lesscomplex FEC encoders and/or decoders.

SUMMARY OF THE DISCLOSURE

Although FEC codes with complex encoders and/or decoders can oftenachieve greater performance than less complex FEC schemes, complex FECencoders and decoders often consume more power. In some applications,power consumption is of little concern, as achieving a high level ofperformance is more important than reducing power consumption. However,in other applications, reducing power consumption is a concern.

The present disclosure provides an optical module configured forprocessing first FEC encoded data produced by a first FEC encoder. Theoptical module has an interface for receiving the first FEC encodeddata, and a second FEC encoder for further coding a subset of the firstFEC encoded data to produce second FEC encoded data. The optical modulealso has an optical modulator for modulating, based on a combination ofthe second FEC encoded data and a remaining portion of the first FECencoded data that is not further coded, an optical signal fortransmission over an optical channel.

In accordance with an embodiment of the disclosure, the second FECencoder is an encoder for an FEC code that has a bit-level trellisrepresentation with a number of states in any section of the bit-leveltrellis representation being less than or equal to 64 states. In thismanner, the second FEC encoder has relatively low complexity (e.g.relatively low transistor count) that can reduce power consumption forthe optical module.

The present disclosure also provides a communication system including atransport circuit having the first FEC encoder, and the optical modulesummarised above. In some implementations, the first FEC encoderimplements a G.975.1/G.709-compliant FEC code.

The present disclosure also provides a corresponding optical module anda corresponding communication system for processing received FEC encodeddata in a manner that is complementary to the optical module and thecommunication system summarised above.

Other aspects and features of the present disclosure will becomeapparent, to those ordinarily skilled in the art, upon review of thefollowing description of the various embodiments of the disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will now be described by way of example with reference tothe attached drawings in which:

FIG. 1 is a block diagram of an example communication system having atransmitting apparatus and a receiving apparatus;

FIG. 2 is a flow chart of an example method for generating andtransmitting an optical signal;

FIG. 3 is a flow chart of an example method for receiving and processingan optical signal;

FIG. 4 is a block diagram of an example transmitting apparatus;

FIG. 5 is a block diagram of an example receiving apparatus;

FIG. 6 is a block diagram of an example transmitting apparatus withset-partitioning mapping;

FIG. 7 is a graph of a set-partitioning constellation for thetransmitting apparatus of FIG. 6;

FIGS. 8 and 9 are graphs of other set partitioning constellations; and

FIG. 10 is a block diagram of a transmitting apparatus and a receivingapparatus that support optical communication in both directions.

DETAILED DESCRIPTION OF EMBODIMENTS

It should be understood at the outset that although illustrativeimplementations of one or more embodiments of the present disclosure areprovided below, the disclosed systems and/or methods may be implementedusing any number of techniques. The disclosure should in no way belimited to the illustrative implementations, drawings, and techniquesillustrated below, including the designs and implementations illustratedand described herein, but may be modified within the scope of theappended claims along with their full scope of equivalents.

Example Communication System

Referring now to FIG. 1, shown is a block diagram of an examplecommunication system 100 having a transmitting apparatus 101 and areceiving apparatus 103. The transmitting apparatus 101 is coupled tothe receiving apparatus 103 through an optical channel 102. Thecommunication system 100 may have other components, but they are notshown for simplicity.

The transmitting apparatus 101 has a transport circuit 110, an opticalmodule 120, and may have other components that are not shown. Thetransport circuit 110 has a first interface 111, a first FEC encoder112, and may have other components that are not shown. The opticalmodule 120 has a second interface 121, a second FEC encoder 122, anoptical modulator 123, and may have other components that are not shown.

The receiving apparatus 103 has an optical module 130, a transportcircuit 140, and may have other components that are not shown. Theoptical module 130 has a first interface 131, a first FEC decoder 132,an optical demodulator 133, and may have other components that are notshown. The transport circuit 140 has a second interface 141, a secondFEC decoder 142, and may have other components that are not shown.

Operation of the communication system 100 will now be described by wayof example. The transmitting apparatus 101 has data to be communicatedto the receiving apparatus 103 over the optical channel 102. In order toenable the data to be recovered at the receiving apparatus 103 even whennoise has been introduced by the optical channel 102, the transmittingapparatus 101 performs FEC coding of the data prior to opticaltransmission.

In particular, the first FEC encoder 112 performs coding of the data toproduce first FEC encoded data, and sends the first FEC encoded datausing the first interface 111. The optical module 120 receives the firstFEC encoded data using the second interface 121, and the second FECencoder 122 further codes a subset of the first FEC encoded data toproduce second FEC encoded data. Finally, the optical modulator 123modulates, based on a combination of the second FEC encoded data and aremaining portion of the first FEC encoded data that is not furthercoded, an optical signal for transmission over the optical channel 102.

According to an embodiment of the disclosure, the second FEC encoder 122is an encoder for an FEC code that has a bit-level trellisrepresentation with a number of states in any section of the bit-leveltrellis representation being less than or equal to 256 states. In thismanner, the second FEC encoder 122 has relatively low complexity (e.g.relatively low transistor count) that can reduce power consumption forthe optical module 120. In some implementations, the number of states inany section of the bit-level trellis representation is 32 states. Otherimplementations with less or more states (e.g. 16 or 64 states) arepossible.

The “bit-level trellis representation” of an FEC code as used hereinimplies an encoder that calculates output bits as a function of inputbits contained within a window of at most N consecutive input bits. Thiswindow “slides” in a direction of the future as encoding progresses,where 2′ is the maximum number of states in any section of the trellis;the trellis states are defined by the sets of possible values of thebits within the current window of at most N consecutive input bits. Theencoding function applied to the windowed bits, to compute the outputbit, may vary as a function of the position of the output bit in thecodeword of the second FEC encoder 122. While this is anencoding-centric description, it is not limited to encoderimplementations. Rather, it defines a class of FEC codes that can beused for encoder implementations and/or decoder implementations.

There are many possibilities for the second FEC encoder 122. In someimplementations, the second FEC encoder 122 implements an extendedHamming code. In other implementations, the second FEC encoder 122implements a convolutional code. Regardless, the second FEC encoder 122has low complexity as noted above. This is in contrast with the firstFEC encoder 112, which in some implementations has higher complexitythan the second FEC encoder 122 because reducing power consumption forthe transport circuit 110 by sacrificing complexity and resultingperformance of the first FEC encoder 112 is not considered to be adesirable trade-off. In some implementations, the first FEC encoder 112implements a G.975.1/G.709-compliant FEC code. An example of this isdescribed in U.S. Pat. No. 8,751,910, which is incorporated by referencein its entirety. However, other hard-decision FECs with good coding gaincan be used.

The optical signal that has been transmitted travels over the opticalchannel 102 and is received by the receiving apparatus 103. The opticalchannel 102 introduces noise into the optical signal. Consequently, theoptical signal that is received is not exactly identical to the opticalsignal that was transmitted. The receiving apparatus 103 processes theoptical signal that is received in order to recover the data.

In particular, the optical demodulator 133 demodulates the opticalsignal to produce received FEC encoded data. The first FEC decoder 132decodes a subset of the received FEC encoded data to produce first FECdecoded data. The subset that is decoded corresponds to the subset thatwas further coded by the second FEC encoder 122 of the transmittingapparatus 101. The optical module 130 sends, to the transport circuit140 using the first interface 131, a combination of the first FECdecoded data and a remaining portion of the received FEC encoded datathat is not decoded. The transport circuit 140 receives the combinationusing the second interface 141, and the second FEC decoder 142 decodesthe combination to recover the data.

The first FEC decoder 132 of the receiving apparatus 103 iscomplementary to the second FEC encoder 122 of the transmittingapparatus 101. Likewise, the second FEC decoder 142 of the receivingapparatus 103 is complementary to the first FEC encoder 112 of thetransmitting apparatus 101.

Therefore, according to an embodiment of the disclosure, the first FECdecoder 132 is a decoder for an FEC code that has a bit-level trellisrepresentation with a number of states in any section of the bit-leveltrellis representation being less than or equal to 256 states. In thismanner, the first FEC decoder 132 has relatively low complexity (e.g.relatively low transistor count) that can reduce power consumption forthe optical module 130. In some implementations, the number of states inany section of the bit-level trellis representation is 32 states. Otherimplementations with less or more states (e.g. 16 or 64 states) arepossible.

Furthermore, in some implementations, the first FEC decoder 132implements an extended Hamming code. In other implementations, the firstFEC decoder 132 implements a convolutional code. Other implementationsare possible to the extent that they are complementary with theimplementations for the second FEC encoder 122 of the transmittingapparatus 101. Regardless, the first FEC decoder 132 has low complexityas noted above. This is in contrast with the second FEC decoder 142,which in some implementations has higher complexity than the first FECdecoder 132 because reducing power consumption for the transport circuit140 by sacrificing complexity and resulting performance of the secondFEC decoder 142 is not considered to be a desirable trade-off. In someimplementations, the second FEC decoder 142 implements aG.975.1/G.709-compliant FEC code. An example of this is described inU.S. Pat. No. 8,751,910, which is incorporated by reference in itsentirety. However, other hard-decision FECs with good coding gain can beused.

The second FEC encoder 122 and the first FEC decoder 132 are present inthe communication system 100 and have been described to have relativelylow complexity. If the complexity were to be increased, for exampleusing an LDPC (Low-Density Parity-Check) code with BICM(Bit-Interleaved-Coded-Modulation), then power consumption for theoptical modules 120, 130 would be higher. If the second FEC encoder 122and the first FEC decoder 132 were to be omitted, then performance wouldbe sub-optimal. Thus, embodiments of the disclosure archive a compromisebetween performance and power consumption.

In some implementations, the transmitting apparatus 101 and thereceiving apparatus 103 have similar or even identical configurations.In particular, the receiving apparatus 103 may also have components forgenerating and transmitting an optical signal, and the transmittingapparatus 101 may also have components for receiving and processing anoptical signal. Thus, optical communication may be supported in bothdirections. In some implementations, for bi-directional communication,there are two optical channels, namely one optical channel per directionof transmission.

In some implementations, the transmitting apparatus 101 and thereceiving apparatus 103 have similar or even identical configurations.In particular, the receiving apparatus 103 may also have components forgenerating and transmitting an optical signal, and the transmittingapparatus 101 may also have components for receiving and processing anoptical signal. Thus, optical communication may be supported in bothdirections. In some implementations, for bi-directional communication,there are two optical channels, namely one optical channel per directionof transmission. For example, as shown in FIG. 10, a transmittingapparatus 101A may include a transmitting transport circuit 110A and atransmitting optical module 120A for transmitting first data over afirst optical channel 102A, and may also include a receiving opticalmodule 130A and a receiving transport circuit 140A for receiving seconddata from a second optical channel 102B, while a a receiving apparatus103A may include a a receiving optical module 130B and a receivingtransport circuit 140B for receiving the first data from the firstoptical channel 102A, and may also include transmitting transportcircuit 110B and a transmitting optical module 120B for transmitting thesecond data over the second optical channel 102B. The transport circuits110A and 110B may be identical to the transport module 110 of FIG. 1,the optical modules 120A ands 120B may be identical to the opticalmodule 120 of FIG. 1, the optical modules 130A ands 130B may beidentical to the optical module 130 of FIG. 1, and the transportcircuits 140A and 140B may be identical to the transport module 140 ofFIG. 1.

Example Methods

Referring now to FIG. 2, shown is a flow chart of an example method forgenerating and transmitting an optical signal. This method may beimplemented by an optical module, for example by the optical module 120of the transmitting apparatus 101 shown in FIG. 1.

At step 201, the optical module receives first FEC encoded data, whichhas been produced by a first FEC encoder. At step 202, the opticalmodule further codes a subset of the first FEC encoded data to producesecond FEC encoded data. According to an embodiment of the disclosure,the coding at step 202 is performed using a second FEC encoder that hasrelatively low complexity (e.g. relatively low transistor count), forexample the second FEC encoder 122 described above with reference toFIG. 1. In this manner, power consumption for the optical module can bereduced as similarly described above with reference to FIG. 1.

In some implementations, the optical module splits the first FEC encodeddata into an LSB (Least Significant Bit) sequence and an MSB (MostSignificant Bit) sequence, such that the LSB sequence is the subset ofthe first FEC encoded data that is further coded by the second FECencoder to produce the second FEC encoded data. The MSB sequence is notfurther coded by the second FEC encoder, which can help to reducecomplexity of the second FEC encoder.

At step 203 the optical module modulates, based on a combination of thesecond FEC encoded data and a remaining portion of the first FEC encodeddata that is not further coded, an optical signal for transmission overan optical channel. For implementations in which there is an MSBsequence that is not further coded by the second FEC encoder, themodulation is based on a combination of the second FEC encoded data andthe MSB sequence.

If at step 204 the transmission is complete, then the method concludes.However, if at step 204 the transmission is not complete because thereis more data to send, then the method loops back to step 201. Steps 201through 203 are repeated until the transmission is complete.

Referring now to FIG. 3, shown is a flow chart of an example method forreceiving and processing an optical signal. This method may beimplemented by an optical module, for example by the optical module 130of the receiving apparatus 103 shown in FIG. 1.

At step 301, the optical module demodulates an optical signal receivedover an optical channel to produce received FEC encoded data. At step302 the optical module decodes a subset of the received FEC encoded datato produce first FEC decoded data. According to an embodiment of thedisclosure, the decoding at step 302 is performed using a first FECdecoder that has relatively low complexity (e.g. relatively lowtransistor count), for example the first FEC decoder 132 described abovewith reference to FIG. 1. In this manner, power consumption for theoptical module can be reduced as similarly described above withreference to FIG. 1.

In some implementations, the optical module splits the received FECencoded data into an LSB sequence and an MSB sequence, such that the LSBsequence is the subset of the received FEC encoded data that is decodedby the first FEC decoder to produce the first FEC decoded data. Thesubset of the received FEC encoded data that is decoded corresponds to asubset that was further coded by a transmitting apparatus. The MSBsequence is not further decoded by the first FEC decoder, which can helpto reduce complexity of the first FEC decoder.

At step 303 the optical module sends, to a transport circuit, acombination of the first FEC decoded data and a remaining portion of thereceived FEC encoded data that is not decoded. The transport circuitthen decodes the combination to recover the data. For implementations inwhich there is an MSB sequence that is not decoded by the first FECdecoder, the optical module sends a combination of the first FEC decodeddata and the MSB sequence to the transport circuit for decoding.

If at step 304 the reception is complete, then the method concludes.However, if at step 304 the reception is not complete because there ismore data to receive, then the method loops back to step 301. Steps 301through 303 are repeated until the reception is complete.

Example Transmitting Apparatus

Referring now to FIG. 4, shown is a block diagram of an exampletransmitting apparatus 400. The transmitting apparatus 400 has a hostcard 410, an optical module 420, and may have other components that arenot shown. The host card 410 has a hard FEC encoder 412, an errordecorelator 413, a framer 414, and may have other components that arenot shown. The optical module 420 has a demultiplexer 421, a soft FECencoder 422, a 2^(M)-point mapper 423, a DSP (Digital Signal Processor)424, an optical modulator 425, and may have other components that arenot shown.

Operation of the transmitting apparatus 400 will now be described by wayof example. The host card 410 receives data to be communicated to areceiving apparatus over an optical channel. In order to enable the datato be recovered at the receiving apparatus even when noise has beenintroduced by the optical channel, the transmitting apparatus 400performs FEC coding of the data prior to optical transmission. Asdescribed in further detail below, the FEC coding is performed by acombination of two FEC encoders: the hard FEC encoder 412 and the softFEC encoder 422.

The hard FEC encoder 412 performs coding of the data to produce hard FECencoded data. The hard FEC encoded data is processed by the errordecorelator 413 before the framer 414 frames the hard FEC encoded datafor transmission to the optical module 420. In some implementations, theerror decorelator 413 performs interleaving, which, when used with acorresponding error decorrelator (e.g. error decorrelator 543 shown inFIG. 5) at a receiving apparatus, serves to randomize positions oferrors at the input of a hard FEC decoder (e.g. hard FEC decoder 544shown in FIG. 5).

In some implementations, the transmission to the optical module 420 isover an OTL (Optical channel Transport Lane) interface. In someimplementations, each frame is an OTUk (Optical channel Transport Unit¹)frame as standardized by ITU (International Telecommunication Union) inG.709/Y.1331 (Feb. 2012) Interfaces for the optical transport network,which is incorporated by reference in its entirety and hereinafterreferred to as “ITU G.709”. Other interfaces and frames are possible andare within the scope of this disclosure. ¹ k (=1,2,3,4) indicates rate(e.g. OTU1=10 Gbps, OTU2=10 Gbps, OTU3=40 Gbps, OTU4=100 Gbps)

The optical module 420 receives the hard FEC encoded data using thesecond interface 121. The demultiplexer 421 splits the hard FEC encodeddata into an LSB sequence and an MSB sequence. The LSB sequence isfurther coded by the soft FEC encoder 422 to produce soft FEC encodeddata. The MSB sequence is not further coded by the soft FEC encoder 422.The 2^(M)-point mapper 423 maps a combination of the soft FEC encodeddata and the MSB sequence into data symbols. The data symbols areprocessed by the DSP 424 prior to the optical modulator 425 modulating,based on the output of the DSP 424, an optical signal for transmissionover an optical channel.

In some implementations, the DSP 424 performs conversion of the symbols.For example, a pair of PAM-4 (Pulse-Amplitude Modulation) symbols can beconverted to a QAM-16 (Quadrature-Amplitude Modulation) symbol. Furtherdetails of such conversion will be discussed later with reference toFIGS. 6 and 7. In some implementations, the DSP 424 also performspre-equalization. In some implementations, the pre-equalization isaccomplished using an FIR (Finite Impulse Response) filter with a smallnumber of taps, for example 2 or 3 taps. In other implementations, thepre-equalization is accomplished using a more complicated pre-equalizerthat compensates for nonlinearities that may be caused by the opticalmodulator 425 or the optical channel. Other implementations arepossible.

According to an embodiment of the disclosure, the soft FEC encoder 422is an encoder that has relatively low complexity (e.g. relatively lowtransistor count) such as the second FEC encoder 122 described abovewith reference to FIG. 1. In this manner, power consumption for theoptical module 420 can be reduced as similarly described above withreference to FIG. 1. This is in contrast with the hard FEC encoder 412,which in some implementations is an encoder that has relatively highcomplexity such as the first FEC encoder 112 described above withreference to FIG. 1.

In some implementations, power consumption in the optical module 420 isreduced with only a minor degradation in end-to-end performance. Thiscan be accomplished by appropriately concatenating the soft FEC encoder422, which has relatively low complexity (e.g. relatively low transistorcount), with the hard FEC encoder 412, which is a powerful hard-decisionouter FEC encoder. In some implementations, the hard FEC encoder 412consumes significantly less power than a powerful soft-decision FEC, andhas an advantage of existing within the host card 410, where sensitivityto power consumption is reduced.

The transmitting apparatus 400 uses an MLC (multi-level coding) for thesoft FEC encoder 422. In some implementations, the constellation of the2^(M)-point mapper 423 is labelled using a mixed Gray/set-partitionedconstellation labelling, such that the LSB sequence in the M-bit labelact as L subset selection bits, which have been encoded by the soft FECencoder 422 in the optical module 420. Note that the soft FEC encoder422 is a block-based code that converts K input bits into N output bits.In some implementations, an elastic FIFO (First-In First-Out) is used tocreate the L-bit output. Furthermore, in some implementations, a memoryused for the FIFO is also used to implement a block interleaver.

Example Receiving Apparatus

Referring now to FIG. 5, shown is a block diagram of an examplereceiving apparatus 500. The receiving apparatus 500 has an opticalmodule 530, a host card 540, and may have other components that are notshown. The optical module 530 has an optical demodulator 531, a DSP 532,a subset LLR (Log-likelihood Ratio) calculator 533, a soft FEC decoder534, a soft FEC encoder 535, a 2^(M)-point demapper 536, a multiplexer537, and may have other components that are not shown. The host card 540has a framer 542, an error decorelator 543, a hard FEC decoder 544, andmay have other components that are not shown.

Operation of the receiving apparatus 500 will now be described by way ofexample. An optical signal that has been transmitted by a transmittingapparatus travels over an optical channel and is received by thereceiving apparatus 500. The optical demodulator 531 demodulates theoptical signal to produce received FEC encoded data. Note that thereceived FEC encoded data has been coded by the transmitting apparatusin order to enable the data to be recovered at the receiving apparatus500 even when noise has been introduced by the optical channel. Asdescribed in further detail below, the FEC decoding is performed by acombination of two FEC decoders: the soft FEC decoder 534 and the hardFEC decoder 544.

The received FEC encoded data is processed by the DSP 532, which in someimplementations performs processing that is complementary to theprocessing of the DSP 424 shown in FIG. 4. Additionally, oralternatively, the DSP 532 calculates soft estimates of the transmittedsymbols. A subset of the received FEC encoded data is decoded by thesoft FEC decoder 534 to produce soft FEC decoded data. To this end, thesubset LLR calculator 533 processes the soft estimates from the DSP 532to calculate magnitude and reliability estimates of the bitscorresponding to the subset of the received FEC encoded data to the softFEC decoder 534. In this example, the subset of the received FEC encodeddata that is decoded by the soft FEC decoder 534 is an LSB sequence.

According to an embodiment of the disclosure, the soft FEC decoder 534is a decoder that has relatively low complexity (e.g. relatively lowtransistor count) such as the first FEC decoder 132 described above withreference to FIG. 1. In this manner, power consumption for the opticalmodule 530 can be reduced as similarly described above with reference toFIG. 1. This is in contrast with the hard FEC decoder 544, which in someimplementations is a decoder that has relatively high complexity such asthe second FEC decoder 142 described above with reference to FIG. 1.

In some implementations, the soft FEC decoder 534 uses soft informationfrom the DSP 532 when performing the decoding. The way in which this isaccomplished is implementation-specific, but in general soft informationenables a more reliable determination of a “most likely” transmittedcodeword. In absence of soft information, the soft FEC decoder 534 wouldfind a candidate codeword with the fewest symbols that differ from thereceived signal; this is called a “hard”-decision metric, becausesymbols are either correct or incorrect. However, in presence of softinformation, a most likely codeword is selected to minimize a “distance”between the soft received values and the candidate codewords; here, apossible metric is a Euclidean distance (i.e., root-sum-of-squares)between the soft values and the candidate codewords). Thus, softinformation allows the soft FEC decoder 534 to use a different“distance” metric to identify the most likely transmitted codeword.

Exploiting soft information from the DSP 532 as described above canincrease coding gain thereby increasing robustness and link operatingmargin of the system. Note that in the absence of a soft-decision FECdecoder in the optical module 530, soft information is discarded, as itcannot be readily transmitted to the host card. Connecting the opticalmodule 530 to the host card 540 through an interface that can carry softinformation involves excessive wiring and is not practical.

The multiplexer 537 combines the soft FEC decoded data and an MSBsequence of the received FEC encoded data, and the combination istransmitted to the host card 540. The MSB sequence is generated by the2^(M)-point demapper 536, which has two inputs: soft estimates of thetransmitted symbols calculated by the DSP 532, and a processed LSBsequence that is generated by the soft FEC encoder 535 coding the softFEC decoded data. By using the processed LSB sequence instead of the LSBsequence obtained directly from the soft estimates of the DSP 532,robustness in the demapping can be increased because the soft FECdecoder 534 may correct some errors prior to the demapping.

In some implementations, the transmission to the host card 540 is overan OTL interface. In some implementations, the transmission includesframes, each of which being an OTUk frame as standardized in ITU G.709.Other interfaces and frames are possible and are within the scope ofthis disclosure.

The host card 540 receives the combination, which is processed by theframer 542 and the error decorrelator 543 prior to being decoded by thehard FEC decoder 544 to recover the data. Residual errors that were notcorrected by the soft FEC decoder 534 are corrected by the hard FECdecoder 544. In some implementations, the framer 542 performs processingthat is complementary to framing that is performed by the transmittingapparatus. In some implementations, the error decorelator 543 performsde-interleaving that is complementary to interleaving that is performedby the transmitting apparatus. The error decorelator 543 serves torandomize positions of any errors at the input to the hard FEC decoder544, which can improve performance.

In some implementations, power consumption in the optical module 530 isreduced with only a minor degradation in end-to-end performance. Thiscan be accomplished by appropriately concatenating the soft FEC decoder534, which has relatively low complexity (e.g. relatively low transistorcount), with the hard FEC decoder 544, which is a powerful hard-decisionouter FEC decoder. In some implementations, the hard FEC decoder 544consumes significantly less power than a powerful soft-decision FEC, andhas an advantage of existing within the host card 540, where sensitivityto power consumption is reduced.

In some implementations, the code rate in the host card 540 is fixed(e.g. R=239/255) by the OTN standard. For such implementations, theconcatenation of the soft FEC decoder 534 and the hard FEC decoder 544can better exploit the strength of the hard FEC decoder 544, while stillbeing able to use soft information available from the DSP 532 in theoptical module 530 as described above. In this manner, a suitablebalance can be achieved between performance and power consumption.

Set Partitioning Examples

Referring now to FIG. 6, shown is a block diagram of an exampletransmitting apparatus 600 with set-partitioning mapping. Thetransmitting apparatus 600 has a first FEC encoder 612, a second FECencoder 622, a PAM-4 mapper 623, a PAM-4 to QAM-16 mapper 624, and mayhave other components that are not shown.

The first FEC encoder 612 encodes data to produce first FEC encodeddata, which in this example is split into an MSB sequence and an LSBsequence. The second FEC encoder 622 further encodes the LSB sequence toproduce second FEC encoded data. The MSB sequence is not further encodedby the second FEC encoder 622.

A combination of the MSB sequence and the second FEC encoded data ismapped to PAM-4 symbols by the PAM-4 mapper 623. Next, each pair of thePAM-4 symbols is mapped to a QAM-16 symbol by the PAM-4 to QAM-16 mapper624. Note that the combination of the PAM-4 mapper 623 and the PAM-4 toQAM-16 mapper 624 constitutes a decomposition of a 2⁴-point 2-D mapperblock. Since a QAM-16 symbol can be represented as a Cartesian productof two PAM-4 symbols, the system can be described in terms of a PAM-4constellation 701 as shown in FIG. 7.

While the transmitting apparatus 600 shown in FIG. 6 is described forQAM-16 modulation, it is to be understood that other QAM modulations arepossible for a multi-dimensional mapper. In some implementations, thesecond FEC encoded data is a stream of set-partitioning bits for symbolsof a PAM-M constellation. Thus, for example, a first symbol mappergenerates PAM-M symbols, and a second symbol mapper for maps pairs ofthe PAM-M symbols into QAM-M² symbols. The transmitting apparatus 600shown in FIG. 6 is a specific case where M=4. However, other values forM are possible and are within the scope of this disclosure.

In some implementations, an optical modulator (not shown) uses amulti-dimensional modulation scheme due to availability of real andcomplex dimensions on each of two orthogonal polarizations of light.However, the dimension of the optical modulator's modulation scheme neednot be the same as the dimension of the multi-dimensional mapper. Forexample, a 4-D optical modulator could be used in concert with a 2-Dmapper in which case two consecutive 2-D symbols are mapped to a single4-D symbol. In a second example, an 8-D symbol mapper may be used inconjunction with a 4-D optical modulator in which case two consecutive4-D symbols are used to transmit a single 8-D symbol.

There are many possibilities for the first FEC encoder 612 and thesecond FEC encoder 622. In some implementations, the first FEC encoder612 is a transport IC encoder that has relatively high complexity suchas the first FEC encoder 112 described above with reference to FIG. 1.In some implementations, the first FEC encoder 612 has a code rate ofr=239/255.In some implementations, the second FEC encoder 622 is amodule soft encoder that has relatively low complexity (e.g. relativelylow transistor count) such as the second FEC encoder 122 described abovewith reference to FIG. 1. In some implementations, the second FECencoder 622 has a code rate of r=1/2.

The first FEC encoder 612 is generally separate from the second FECencoder 622. For instance, some implementations, the first FEC encoder612 resides on a transport circuit of a host card, while the second FECencoder 622 resides in an optical module that interfaces with thetransport circuit. In other implementations, the first FEC encoder 612resides within the same optical module as the second FEC encoder 622,but on a separate chip. Regardless of location, in some implementations,the first FEC encoder 612 is relied upon to protect bits that are notcoded by the second FEC encoder 622 and to improve the BER (Bit ErrorRate) of bits that are coded by the second FEC encoder.

In some implementations, the second FEC encoded data is a stream ofset-partitioning bits for symbols of a DP-DQPSK (Dual-PolarizationDifferential Quadrature-Phase-Shift-Keying) constellation. DP-DQPSK isnaturally described in terms of its 4-D constellation. Thisconstellation includes 16 points, which can be represented by 16possible 4-tuples in Euclidean space: (+/−1, +/−1, +/−1, +/−1). Notethat different multi-level-coding schemes are possible, with differentnumbers of bits (from the 4-bit label) being set-partitioning bits codedby a soft FEC. Examples are described below with reference to FIGS. 8and 9.

Referring now to FIG. 8, shown is a graph of a constellation 801, 802 inwhich only one of the four bits is a set-partitioning bit. Note thatvertices represented by the same symbol (i.e., crosses or circles) havea common LSB, and that the constellation 801, 802 is a 4-D constellationrepresented by two projections: a first 3-D constellation 801 withx₄=+1, and a second 3-D constellation 802 with x₄=−1.

When two of the four bits are set-partitioning bits, it is possible totreat each polarization as an independent DQPSK scheme. Referring now toFIG. 9, shown is a graph of a constellation 901 in which one of the twobits in the DQPSK constellation is a set-partitioning bit.

Numerous modifications and variations of the present disclosure arepossible in light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims, the disclosuremay be practised otherwise than as specifically described herein.

We claim:
 1. A method of operating a communication system, the methodcomprising: coding, by a transmit transport circuit of a transmittingapparatus, first forward error correction (FEC) encoded data using afirst FEC encoder; coding, by a transmit optical module of thetransmitting apparatus, a subset of the first FEC encoded data toproduce second FEC encoded data using a second FEC encoder; modulating,by the transmit optical module, an optical signal for transmission overan optical channel based on a combination of the second FEC encoded dataand a remaining portion of the first FEC encoded data that is notfurther coded; demodulating, by a receive optical module of a receivingapparatus, the optical signal received over the optical channel toproduce received FEC encoded data; decoding, by the receive opticalmodule, a subset of the received FEC encoded data to produce first FECdecoded data using a first FEC decoder; and decoding, by a receivetransport circuit of the receiving apparatus, a combination of the firstFEC decoded data and a remaining portion of the received FEC encodeddata that is not decoded to produce recovered data using a second FECdecoder.
 2. The method of claim 1 wherein the second FEC encoder is anencoder for an FEC code that has a bit-level trellis representation witha number of states; wherein the first FEC decoder is a decoder for anFEC code that has a bit-level trellis representation with a number ofstates; and wherein the number of states in any section of the bit-leveltrellis representation of the FEC code for the second FEC encoder or thefirst FEC decoder is less than or equal to 256 states.
 3. The method ofclaim 1 wherein the second FEC encoder or the first FEC decoderimplements an extended Hamming code or a convolutional code; and whereinthe first FEC encoder or the second FEC decoder implements aG.975.1/G.709-compliant FEC code.
 4. The method of claim 1 wherein thefirst FEC encoder comprises a hard FEC encoder, the second FEC encodercomprises a soft FEC encoder, the first FEC decoder comprises a soft FECdecoder, and the second FEC decoder comprises a hard FEC decoder.
 5. Themethod of claim 1 further comprising sending, by a first transmitinterface of the transmit transport circuit, the first FEC encoded datato the transmit optical module; receiving, by a second transmitinterface of the transmit optical module, the first FEC encoded datafrom the first transmit interface; sending, by a first receive interfaceof the receive optical module, the combination of the first FEC decodeddata and the remaining portion of the received FEC encoded data that isnot decoded to the receive transport circuit; and receiving, by a secondreceive interface of the receive transport circuit, the combination ofthe first FEC decoded data and the remaining portion of the received FECencoded data that is not decoded from the first receive interface. 6.The method of claim 5 wherein the first transmit interface, the secondtransmit interface, the first receive interface or the second receiveinterface comprises an optical channel transport lane interface or anITU G.709 interface.
 7. The method of claim 1 wherein modulating, by thetransmit optical module, the optical signal comprises modulating, by anoptical modulator of the transmit optical module, the optical signal fortransmission over the optical channel based on a combination of thesecond FEC encoded data and the remaining portion of the first FECencoded data that is not further coded; and wherein demodulating, by thereceive optical module, the optical signal comprises demodulating, by anoptical demodulator of the receive optical module, the optical signalreceived over the optical channel to produce the received FEC encoded.8. The method of claim 1 wherein the first FEC decoder is configured tobe complementary to the second FEC encoder, and wherein the second DECdecoder is configured to be complementary to the first FEC encoder. 9.The method of claim 1 further comprising generating and transmitting, bythe receiving apparatus, an optical signal; and receiving andprocessing, by the transmitting apparatus, an optical signal; whereinthe transmitting apparatus and the receiving apparatus are configuredfor bi-directional communication.
 10. The method of claim 9 wherein theoptical channel comprises two optical channels, including a firstoptical channel configured for transmission of an optical signal fromthe transmitting apparatus to the receiving apparatus and a secondoptical channel configured for transmission from the receiving apparatusto the transmitting apparatus.
 11. The method of claim 1 wherein thetransmit transport circuit is configured within a transmit host card;and wherein the receive transport circuit is configured within a receivehost card.
 12. The method of claim 1 further comprising: processing, bya transmit error decorrelator of the transmitting apparatus, the firstFEC encoded data; framing, by a transmit framer of the transmittingapparatus, the first FEC encoded data for transmission to the transmitoptical module; deframing, by a receive framer of the receivingapparatus, the combination of the first FEC decoded data and theremaining portion of the received FEC encoded data that is not decoded;and processing, by a receive error decorrelator of the receivingapparatus, the combination of the first FEC decoded data and theremaining portion of the received FEC encoded data that is not decoded.13. The method of claim 12 wherein the transmit transport circuit isconfigured within a transmit host card, the transmit host cardcomprising the first FEC encoder, the transmit error decorrelator, andthe transmit framer; and wherein the receive transport circuit isconfigured within a receive host card, the receive host card comprisingthe receive framer, the receive error decorrelator, and the second FECdecoder.
 14. The method of claim 12 further comprising interleaving, bythe transmit error decorrelator, the first FEC encoded data; anddeinterleaving, by the receive error decorrelator, the combination ofthe first FEC decoded data and the remaining portion of the received FECencoded data that is not decoded; wherein the interleaving, by thetransmit error decorrelator, and the deinterleaving, by the receiveerror decorrelator, randomizes positions of errors in the opticalsignal.
 15. The method of claim 1 further comprising splitting, by ademultiplexer of the transmit optical module, the first FEC encoded datainto a Least Significant Bit (LSB) sequence and a Most Significant Bit(MSB) sequence; wherein the LSB sequence is the subset of the first FECencoded data that is further coded by the second FEC encoder to producethe second FEC encoded data.
 16. The method of claim 1 furthercomprising mapping into symbols, by a symbol mapper of the transmitoptical module, the combination of the second FEC encoded data and theremaining portion of the first FEC encoded data that is not furthercoded; and demapping, by a symbol demapper of the receive opticalmodule, the symbols.
 17. The method of claim 16 wherein mapping intosymbols, by the symbol mapper, comprises: mapping into first symbols, bya first symbol mapper of the transmit optical module, the combination ofthe second FEC encoded data and the remaining portion of the first FECencoded data that is not further coded; and mapping into second symbols,by a second symbol mapper of the transmit optical module, the firstsymbols.
 18. The method of claim 16 wherein mapping into symbols, by thesymbol mapper, comprises: mapping into PAM-M symbols, by a first symbolmapper of the transmit optical module, the combination of the second FECencoded data and the remaining portion of the first FEC encoded datathat is not further coded; and mapping into QAM-M2 symbols, by a secondsymbol mapper of the transmit optical module, the PAM-M symbols.
 19. Themethod of claim 16 wherein the second FEC encoded data is a stream ofset-partitioning bits for symbols of a DP-DQPSK (Dual-PolarizationDifferential Quadrature-Phase-Shift-Keying) constellation or wherein thesecond FEC encoded data is a stream of set-partitioning bits for symbolsof a PAM-M (Pulse-Amplitude Modulation) constellation.
 20. The method ofclaim 16 further comprising processing, by a transmit digital signalprocesser (DSP) of the transmit optical module, the symbols; andmodulating, by an optical modulator of the transmit optical module, theoptical signal based on the symbols for transmission over the opticalchannel.
 21. The method of claim 20 further comprising converting, bythe transmit DSP, the symbols; and de-converting, by a receive DSP ofthe receive optical module, the symbols.
 22. The method of claim 16further comprising calculating, by a receive digital signal processor(DSP) of the receive optical module, soft estimates of the symbols; andwherein decoding, by the receive optical module, the subset of thereceived FEC encoded data using the first FEC decoder comprisesdecoding, by the first FEC decoder, the subset of the received FECencoded data using the soft estimates.
 23. The method of claim 22further comprising processing, by a subset LLR calculator of the receiveoptical module, the soft estimates of the symbols to calculate magnitudeand reliability estimates of bits corresponding to the subset of thereceived FEC encoded data; wherein decoding, by the receive opticalmodule, the subset of the received FEC encoded data using the first FECdecoder comprises decoding, by the first FEC decoder, the subset of thereceived FEC encoded data using the magnitude and reliability estimates.24. The method of claim 22 wherein decoding, by the receive opticalmodule, the subset of the received FEC encoded data using the first FECdecoder comprises decoding, by the first FEC decoder, the subset of thereceived FEC encoded data using the soft estimates by determining a mostlikely transmitted codeword according to a distance metric.
 25. Themethod of claim 24 wherein the distance metric is a Euclidean distancemetric.
 26. The method of claim 22 further comprising generating, by asymbol demapper of the receive optical module, a Most Significant Bit(MSB) sequence of the received FEC encoded data using the softestimates; and combining, by a multiplexer of the receive opticalmodule, the first FEC decoded data and the MSB sequence for transmissionto a receive host card including the receive transport circuit.
 27. Themethod of claim 26 further comprising coding, by an FEC encoder of thereceive optical module, the first FEC decoded data to produce aprocessed Least Significant Bit (LSB) sequence; wherein generating, bythe symbol demapper, the MSB sequence comprises generating, by thesymbol demapper, the MSB sequence using the soft estimates and theprocessed LSB sequence.
 28. The method of claim 1 further comprisingcombining, by a multiplexer of the receive optical module, the first FECdecoded data and the remaining portion of the received FEC encoded datathat is not decoded.
 29. The method of claim 28 where the subset of thereceived FEC encoded data is a Least Significant Bit (LSB) sequence andthe remaining portion of the received FEC encoded data is a MostSignificant Bit (MSB) sequence.
 30. A method of operating acommunication system, the method comprising: encoding, by a transmittransport circuit of a transmitting apparatus, first forward errorcorrection (FEC) encoded data using a first FEC encoder; interleaving,by the transmit transport circuit, the first FEC encoded data; framing,by the transmit transport circuit, the first FEC encoded data;splitting, by a transmit optical module of the transmitting apparatus,the first FEC encoded data into a Least Significant Bit (LSB) sequenceand a Most Significant Bit (MSB) sequence; encoding, by the transmitoptical module, the LSB sequence to produce second FEC encoded datausing a second FEC encoder; mapping, by the transmit optical module, thecombination of the second FEC encoded data and the MSB sequence intosymbols; modulating, by the transmit optical module, an optical signalbased on the symbols for transmission over an optical channel; anddemodulating, by a receive optical module of a receiving apparatus, theoptical signal to produce received FEC encoded data; calculating, by thereceive optical module, soft estimates; decoding, by the receive opticalmodule, an LSB sequence of the received FEC encoded data using a firstFEC decoder with the soft estimates to produce first FEC decoded data;encoding, by the receive optical module, the first FEC decoded datausing an FEC encoder to produce a processed LSB sequence producing, bythe receive optical module, an MSB sequence of the received FEC encodeddata using the soft estimates and the processed LSB sequence; combining,by the receive optical module, the first FEC decoded data and the MSBsequence; deframing, by a receive transport circuit of the receivingapparatus, the combination of the first FEC decoded data and the MSBsequence; de-interleaving, by the receive transport circuit, thecombination of the first FEC decoded data and the MSB sequence;decoding, by the receive transport circuit, the combination of the firstFEC decoded data and the MSB sequence using a second FEC decoder toproduce recovered data.
 31. A method of operating a communicationsystem, the method comprising: encoding, by a first forward errorcorrection (FEC) encoder of a transmit host card within a transmittingapparatus, first FEC encoded data; interleaving, by a transmit errordecorrelator of the transmit host card, the first FEC encoded data;framing, by a framer of the transmit host card, the first FEC encodeddata; splitting, by a demultiplexer of transmit optical module withinthe transmitting apparatus, the first FEC encoded data into a LeastSignificant Bit (LSB) sequence and a Most Significant Bit (MSB)sequence; encoding, by a second FEC encoder of the transmit opticalmodule, the LSB sequence to produce second FEC encoded data; mapping, bya symbol mapper of the transmit optical module, the combination of thesecond FEC encoded data and the MSB sequence into first symbols;converting, by a transmit digital signal processer (DSP) of the transmitoptical module, the first symbols into second symbols; modulating, by anoptical modulator of the transmit optical module, an optical signalbased on the second symbols for transmission over an optical channel;demodulating, by an optical demodulator of a receive optical modulewithin a receiving apparatus, the optical signal to produce received FECencoded data; de-interleaving, by a receive DSP of the receive opticalmodule, the received FEC encoded data; calculating, by the receive DSP,soft estimates; processing, by a subset Log-Likelihood Ratio (LLR)calculator of the receive optical module, the soft estimates; decoding,by a first FEC decoder of the receive optical module, an LSB sequence ofthe received FEC encoded data using the soft estimates to produce firstFEC decoded data; encoding, by an FEC encoder of the receive opticalmodule, the first FEC decoded data to produce a processed LSB sequence;generating, by a symbol demapper of the receive optical module, an MSBsequence of the received FEC encoded data using the soft estimates andthe processed LSB sequence; combining, by a multiplexer of the receiveoptical module, the first FEC decoded data and the MSB sequence;deframing, by a deframer of a receive host card within the receivingapparatus, the combination of the first FEC decoded data and the MSBsequence; de-interleaving, by a receive error decorrelator of thereceive host card, the combination of the first FEC decoded data and theMSB sequence; and decoding, by a second FEC decoder of the receive hostcard, the combination of the first FEC decoded data and the MSB sequenceto produce recovered data.